End-contact type thermal head and manufacturing method therefor

Abstract: The present invention relates to a corner head type thermal head provided with a slope (18). The slope (18) is formed from a predetermined position on a top face of a glaze layer (11) provided on a flat surface of the substrate (10) to a substrate side (17) near a heater area (13) provided on the top of the glaze layer (11). A resistance film layer (12) and a common electrode (15) are provided on the slope (18). At least an intersection part of the slope (18) and the substrate side (17) is formed as a convexly curved surface.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to a thermal head and more particularly to a corner head type thermal head improved in printing efficiency and a manufacturing method therefor. 2. Description of the Related Art In using a thermal head, it is necessary to concentrate pressure on a ribbon, print paper, and a platen in a heater area for supporting print on rough paper and improving printing efficiency. For this purpose, hitherto, a near edge type thermal head with a heater area provided near the edge of the thermal head has been installed so as to be inclined against a platen for concentrating pressure against a ribbon and print paper on the heater area and its vicinity. Such a near edge type thermal head is disclosed, for example, in Japanese Utility Model Publication No.Hei 4-46929. An example of the head is shown in Figures 13 and 14. Figure 13 shows a sectional view of the near edge type thermal head, wherein a glaze layer is formed on a substrate 10 and a resistance film layer 12, electrodes 14 and 15, and a protective film are provided thereon. To use the thermal head, print paper 30 is placed under a platen 51 and from under the print paper, a ribbon 31 is pressed onto the print paper 30 by the thermal head 50, as shown in Figure 14. At this time, the thermal head 50 is supported by a carriage so that it is inclined against the platen 41. As shown in Figure 13, a slope 18 constituted by a part of the glaze layer 11 and a part of the substrate 10 is formed on the edge of the thermal head 50 for facilitating passage of the ribbon 31 when the thermal head 50 is inclined. However, the intersecting part 55 of the slope 18 and the top surface of the glaze layer 11, namely, the corner part 55 is not applied on a heater area 13. The near edge type thermal head has a heater part having a small curvature and the heater area formed so as not to lie across the corner part 55, thus the inclination angle of the thermal head against the platen 51 cannot be made large. The angle is from several degrees to less than 10 degrees at most. Therefore, concentration of pressure on the ribbon and print paper cannot be made so high with the result that printing efficiency is insufficient and a good print on rough paper cannot be provided. To solve the problem, a corner head type thermal head is used with the above-mentioned corner part formed in a glaze layer located near the edge of the thermal head and a heater area formed so as to lie across the corner part. Examples of such a corner head type thermal head are shown in Figures 15 and 16(a),(b). Figure 15 shows a sectional view of an example of the conventional corner head type thermal head, wherein a glaze layer 11 is formed on a substrate 10 and a resistance film layer 12 is formed on the glaze layer 11. The glaze layer 11 in the example is of partial glaze type and has the sectional form like a mountain. A heater area 13 with a predetermined part generating heat when the print operation is performed is formed on the top of the mountain. A slope 18 is provided from the heater area 13 to the side 17 of the substrate 10 edge near the heater area 13. A common electrode 15 is provided on the slope 18. A discrete electrode 14 for supplying a current to the predetermined part of the heater area 13 in conjunction with the common electrode 15 is formed in an area on the resistance film layer 12, the area facing the common electrode 15 with the heater area 13 between. In the example, current flows from the common electrode 15 via the resistance film layer 12 of the heater area 13 into the discrete electrode 14. As seen in Figure 15, the slope 18 end on the side of the glaze layer 11, namely, the corner part 55 is formed so as to be applied on the heater area 13. Therefore, the heater area 13 is formed so as to lie across the corner part 55. A protective film 16 is formed on the top layer. Figures 16(a),(b) show another example of the conventional corner head type thermal head. Parts identical with or similar to those previously described with reference to Figure 15 are denoted by the same reference numerals in Figures 16(a),(b) and will not be discussed again. Figure 16 (a) shows a sectional view of the thermal head in the example. A discrete electrode 14 and a common electrode 15 are provided on the same side with respect to a heater area 13 and a turned common electrode 45 is provided facing the discrete electrode 14 and the common electrode 15 with the heater area 13 between. Their arrangement is shown as a partial plan view in Figure 16 (b), wherein supply voltage is supplied to the common electrodes 15 and current flows into the discrete electrodes 14 via the heater area 13 and the turned common electrodes 45. Next, Figure 17 shows a use example of the conventional corner head type thermal head. In Figure 17, the corner head type thermal head 50 is installed so as to be inclined against a platen 51 for concentrating pressure against a ribbon 31 and print paper 30 on the heater area 13. In the example, the inclination angle of the corner head type thermal head can be made larger than that of the near end type thermal head; normally, it can be set to about 10 degrees to 35 degrees. The curvature of the heater part of the corner head type thermal head can also be made larger than that of the near end type thermal head. Thus, the concentration of pressure is raised, improving the printing efficiency. However, since the slope 18 is flat, the intersecting part 20 of the side of the thermal head and the slope 18 has a corner. The curvature of the heater part becomes large, the head sinks into the ribbon 31 and print paper 30 deeply, and the inclination angle increases, so that the intersecting part 20 approaches the ribbon 31, etc., compared with the near edge type thermal head. Thus, the ribbon 31 is in sliding contact with the top of the intersecting part 20 and is worn or cut. Dirty print occurs on print paper 30 because of powder from the ribbon 31. Further, if thermosensible paper is used as print paper 30, it is also in sliding contact with the top of the intersecting part 20, causing pressure rubbing of the paper, so that it causes a mark. In the example of the conventional corner head type thermal head shown in Figure 15, the width of the slope 18, L, is about 200 µm. Therefore, the width of the common electrode 15 formed in the part is limited to 200 µm or less. If the common electrode 15 is made thicker, a disadvantage such as catching of the ribbon occurs and the thickness is also limited. Thus, if the heater area is lengthened or the number of heaters is increased in the conventional corner head type thermal head, the resistance value of the common electrode 15 becomes large and the voltage drop at the parts far from the part to which supply voltage is supplied becomes large, degrading the printing quality. On the other hand, in the example of the conventional corner head type thermal head shown in Figure 16, a power supply is connected to each of the common electrodes 13 individually, thus the voltage drop can be reduced and the problem in the example in Figure 15 can be dealt with. However, in the example shown in Figure 16, the substantial area of the heater area 13 corresponding to one picture element becomes twice that in the example shown in Figure 15; the corner head type thermal head in the example shown in Figure 16 is not applicable to an application where a fine pattern is required. EP-A-0 497 551 shows a thermal printing head having a substrate, a glaze layer, a resistance film layer, electrodes facing each other, and a heater area provided between the electrodes. A slope formed as a convexly curved surface is provided from a predetermined position on a top face of said glaze layer to a substrate side. However, the intersection of the slope and the substrate side is not curved. JP-A-4-169 247 discloses several thermal printing heads each of them includes a substrate, a glaze layer, a resistance film layer, electrodes, and a protective film. The substrate of a first thermal head has a convexly curved corner part. The glaze layer is provided on the boundary between the flat top surface of the substrate and the corner part so that the glaze layer extends onto the convexly curved corner part to cover it nearly completely. Depositing the glaze layer onto the slope of the substrate is rather difficult because the glass paste printed on the boundary tends to flow toward the side surface of the substrate. Hence, it is difficult to obtain a thermal printing head which is exactly formed as designed. A second thermal head includes the same substrate as that of the first thermal head but has an additional layer provided on the convexly curved corner part. The glaze layer is deposited on the top surface of the substrate and on the additional layer and extends to the slope formed by the convexly curved corner part. Hence, the tendency of the glass paste to flow toward the slope during heat treatment is reduced but not prevented. A third thermal printing head has a glaze layer formed only on the flat upper surface of the substrate. However, this thermal head has no smoothly rounded corner portion but a sharp edge near the heater area. This sharp edge causes wearing and possibly cutting a ribbon during printing operation. A fourth thermal head includes a substrate having a straight slope at the corner portion instead of a convexly curved portion or a sharp edge. The straight slope is covered by an additional layer which seems to be used as a barrier for preventing the glass paste from flowing toward the straight slope during heat treatment. However, the glaze layer is off set from the slope portion so that the contour of the corner part is not convexly but concavely formed near the heater area. SUMMARY OF THE INVENTION It is therefore an object of the invention to provide a corner head type thermal head to prevent dirty printing and wearing and cutting of a ribbon without losing the advantages of a corner head and a large corner head type thermal head to reduce the resistance value of common electrodes and maintain low costs in order to allow the containing of a large number of heaters. To these ends, according to the present invention, there is provided a corner head type thermal head as described in claim 1 and a method for manufacturing such a thermal head as described in claim 3. BRIEF DESCRIPTION OF THE DRAWINGS In the accompanying drawings: Figure 1 is a sectional view of a corner head type thermal head according to a first embodiment of the invention; Figure 2 is a sectional view of a corner head type thermal head according to a second embodiment of the invention; Figure 3 is a sectional view of a corner head type thermal head according to a third embodiment of the invention; Figure 4 is a sectional view of a corner head type thermal head according to a fourth embodiment of the invention; Figure 5 is a process drawing showing a step in a manufacturing method of the corner head type thermal head in Figure 1; Figure 6 is a process drawing showing a step in the manufacturing method of the corner head type thermal head in Figure 1; Figure 7 is a process drawing showing a step in the manufacturing method of the corner head type thermal head in Figure 1; Figure 8 is a process drawing showing a step in the manufacturing method of the corner head type thermal head in Figure 1; Figure 9 is a process drawing showing a step in the manufacturing method of the corner head type thermal head in Figure 1; Figure 10 is an illustration of a printing mechanism of a printer using the corner head type thermal head in Figure 1; Figure 11(a) is an illustration of a method of manufacturing a corner head type thermal head of full glaze type according to the invention; Figure 11(b) is a sectional view of a corner head type thermal head of full glaze type according to the invention; Figure 12(a) is an illustration of a method of manufacturing the corner head type thermal head in Figure 2; Figure 12(b) is a sectional view of a corner head type thermal head manufactured by the method of Figure 12(a); Figure 13 is a sectional view of an example of a conventional near edge type thermal head; Figure 14 is an illustration of a printing mechanism of a printer using the near edge type thermal head in Figure 13; Figure 15 is a sectional view of an example of a conventional corner head type thermal head; Figure 16(a) is a sectional view of another example of the conventional corner head type thermal head; Figure 16(b) is a plan view of another example of the conventional corner head type thermal head; and Figure 17 is an illustration of a printing mechanism of a printer using the conventional corner head type thermal head. DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the accompanying drawings, there are shown preferred embodiments of the invention. First embodiment: Figure 1 shows a sectional view of a first embodiment of the invention. Parts identical with or similar to those previously described with reference to Figures 32-34 are denoted by the same reference numerals in Figure 1. In Figure 1, a glaze layer 11 is formed near the end face on a substrate 10 and a resistance film layer 12 is formed on the glaze layer 11. The glaze layer 11 in the embodiment has the sectional form like a mountain. A discrete electrode 14 and a common electrode 15 are provided at a given interval to form a heater area 13 on the top of the mountain shaped portion, namely, the corner part 55. The resistance film layer 12, the discrete electrode 14, and the common electrode 15 are covered with a protective film 16. The embodiment shown in Figure 1 is characterized by the fact that a slope 18 is formed from the mountain top (corner part 55) in the heater area 13 to the side 17 of the end face of the substrate 10 and that the entire slope 18 is formed as a convexly curved surface. Second embodiment: Figure 2 shows a sectional view of a second embodiment of the invention. Parts identical with or similar to those previously described with reference to Figure 1 are denoted by the same reference numerals in Figure 2 and will not be discussed again. The embodiment shown in Figure 2 is characterized by the fact that an intersecting part 20 of a slope 18 from a heater area 13 to the side 17 of a substrate 10 and the side 17 is formed as a convexly curved surface. As described above, in the first and second embodiments shown in Figures 1 and 2, the entire slope 18 or the intersecting part 20 of the slope 18 and the side 17 is formed as a convexly curved surface, so that if a ribbon is in sliding contact with the portion, the ribbon is not worn or cut. The embodiments shown above can be selected according to the application of the thermal heads. Ninth embodiment: A third embodiment of the invention is shown in Figure 3. The embodiment shown in Figure 3 is characterized by the fact that the corner head type thermal head with the entire slope 18 formed as a convexly curved surface according to the first embodiment has a reinforcement conductor 35 along a common electrode 15 embedded below a resistance film layer 12 of a slope 18 as in the third embodiment. Fourth embodiment: A tenth embodiment of the invention is shown in Figure 4. The embodiment shown in Figure 4 is characterized by the fact that the corner head type thermal head with the intersecting part 20 of the slope 18 and the side 17 of the substrate 10 formed as a convexly curved surface according to the second embodiment has a reinforcement conductor 35 along a common electrode 15 embedded below a resistance film layer 12 of a slope 18. Next, a method of manufacturing the corner head type thermal head in the first embodiment is described. Figures 5 to 9 show the manufacturing steps of the thermal head. In the step shown in Figure 5, a mountain-like glaze layer 11 is formed on the top surface of a substrate 10. In the step shown in Figure 6, the glaze layer 11 and the substrate 10 are half cut with a blade 25 so as to leave a part of the glaze layer 11 from the top surface of the glaze layer 11 to the substrate 10. The blade 25 has a slant part 26 as a part of the side, the slant part 26 being formed as a concavely curved surface. To half cut them, the glaze layer 11 is cut with the slant part 26, thereby forming a groove 21 (Figure 7) having the inclined side constituted of the glaze layer 11 and the substrate 10. The embodiment is characterized in that the glaze layer 11 side of the groove 21 formed by the half cutting is formed as a convexly curved surface. In the step shown in Figure 7, the substrate 10 where the groove 21 is formed by the half cutting is heat-treated. Burrs produced on the top 22 of the glaze layer 11 by the half cutting are removed by the heat treatment for rounding the top 22. The top 22 will become the corner part 55 shown in Figure 1. The glaze cut part on the face of the groove 21 formed by the half cutting is low in smoothness, but the smoothness of the face is also improved by the heat treatment. Thus, subsequent pattern formation is facilitated. Although not shown, the substrate 10 is a large substrate from which a large number of thermal heads can be provided, and a plurality of grooves 21 are formed. In the step shown in Figure 8, films of a resistance film layer 12, a discrete electrode 14, and a common electrode are formed by a photo-lithography process. In this case, the discrete electrode 14 and the common electrode 15 are spaced out to form a heater area 13 in the vicinity of the top of the glaze layer 11. Further, a protective film 16 is formed to provide the substrate before division as shown in Figure 8. Lastly, the substrate is cut and divided along the A-A line shown in Figure 8 to provide a separate thermal head shown in Figure 9. The corner head type thermal head is now complete. In the manufacturing method of the corner head type thermal head according to the invention described above, grooves 21 are formed by half cutting and a predetermined pattern is formed, then separate thermal heads are produced by cutting or cracking, whereby a large number of thermal heads can be prepared easily and simultaneously. Figure 10 shows a printing mechanism of a printer using the corner head type thermal printer according to the first embodiment manufactured by the inventive method as described with regard to figures 5 to 9. Print paper 30 and a ribbon 31 are put between the glaze layer 11 and a platen 51 and printing is performed by heat generation of the heater area 13. The slope 18 is formed at the end of the thermal head and the entire slope 18 or the intersecting part of the slope 18 and the side 17 of the substrate 10 is formed as a convexly curved surface, so that the ribbon 31 is in sliding contact with the smooth face and can be prevented from being worn or cut. Fifth embodiment: In the above-mentioned embodiments, examples in which a substrate of partial glaze type is used are discussed, but the method of manufacturing the thermal head according to the invention can also be applied to cases where a substrate of full glaze type is used. An example thereof is shown in Figure 11(a),(b). As shown in Figure 11 (a), a glaze layer 11 is formed fully on a substrate 10. The full glaze layer 11 is half cut with the above-mentioned blade 25, thereby forming a thermal head of the full glaze type with the end having a slope formed as a convexly curved surface, as shown in Figure 11 (b). Next, to manufacture the corner head type thermal head shown in Figure 2, a blade 25 of the form as shown in Figure 12 (a) may be used for half cutting. A slant part 26 of the blade 25 consists of a nose 27 formed as a concavely curved surface and a linear part 28. The rest of the steps are the same as the steps shown in Figures 7 to 9 and the thermal head shown in Figure 12 (b) is thus manufactured; it is the same as the thermal head shown in Figure 2.

Claims: A corner head type thermal head comprising: a glaze layer (11) provided on a flat surface of a substrate (10); a slope (18) formed from a predetermined position on a top face of said glaze layer (11) to a substrate side (17); a convex corner part (55) formed by intersection of said slope (18) and the top face of said glaze layer (11); a resistance film layer (12) provided on said slope (18) and on said glaze layer (11); a heater area (13) formed lying across a top of said corner part (55); a common electrode (15) provided on said resistance film layer (12) in said slope area (18); and a discrete electrode (14) provided on said resistance film layer (12) in an area facing said common electrode (15) with said heater area (13) between for causing a current to flow into a predetermined portion of said heater area (13) in association with said common electrode (15); characterized , in that at least an intersection part of said slope (18) and said substrate side (17) is formed as a convexly curved surface. The corner head type thermal head as claimed in claim 1; characterized , in that a full face of said slope (18) is formed as a convexly curved surface. A method of manufacturing a corner head type thermal head, comprising the steps of: forming a glaze layer (11) on a flat surface of a substrate (10); preparing a cutting blade (25) having a slant part (26) with at least a neighborhood of a nose formed as a concavely curved surface; half cutting said glaze layer (11) and said substrate (10) with said cutting blade (25) downward from a predetermined position on a top surface of said glaze layer (11) for forming a groove (21) one side of which forms a slope being constituted of said glaze layer (11) and said substrate (10), said slope being formed as a convexly curved surface in at least a vicinity of a bottom of said groove (21); heat-treating said substrate (10) and said glaze layer (11); forming a resistance layer (12), a common electrode (15) and a discrete electrode (14), and a protective film (16) on said glaze layer (11) and on said slope; and cutting said substrate on both sides of said groove (21). The method as claimed in claim 3, characterized , in that the entire slant part of said cutting blade (25) is formed as a concavely curved surface.

Inventors: OHNISHI HIROAKI, OHNISHI, HIROAKI
Applicants: ROHM CO LTD, ROHM CO., LTD.
Application year: 1994
Main classification: B41J2
IPC codes: B41J2